This PR improves the error message when no library suggestions engine is
registered to recommend importing `Lean.LibrarySuggestions.Default` for
the built-in engine.
**Before:**
```
No library suggestions engine registered. (Note that Lean does not provide a default library suggestions engine, these must be provided by a downstream library, and configured using `set_library_suggestions`.)
```
**After:**
```
No library suggestions engine registered. (Add `import Lean.LibrarySuggestions.Default` to use Lean's built-in engine, or use `set_library_suggestions` to configure a custom one.)
```
🤖 Prepared with Claude Code
Co-authored-by: Claude <noreply@anthropic.com>
a0c8404ab8